The invention relates to a current source used as part of the control circuitry for display devices, and particularly display devices having current-addressed pixels. Such display devices may comprise an array of electroluminescent display pixels arranged in rows and columns.
Matrix display devices employing electroluminescent, light-emitting, display elements are well known. The display elements may comprise organic thin film electroluminescent elements, for example using polymer materials, or else light emitting diodes (LEDs) using traditional III-V semiconductor compounds. Recent developments in organic electroluminescent materials, particularly polymer materials, have demonstrated their ability to be used practically for video display devices. These materials typically comprise one or more layers of a semiconducting conjugated polymer sandwiched between a pair of electrodes, one of which is transparent and the other of which is of a material suitable for injecting holes or electrons into the polymer layer.
The polymer material can be fabricated using a CVD process, or simply by a spin coating technique using a solution of a soluble conjugated polymer. Organic electroluminescent materials exhibit diode-like I-V properties, so that they are capable of providing both a display function and a switching function, and can therefore be used in passive type displays. Alternatively, these materials may be used for active matrix display devices, with each pixel comprising a display element and a switching device for controlling the current through the display element. Examples of an active matrix electroluminescent display are described in EP-A-0653741 and U.S. Pat. No. 5,670,792, the contents of which are incorporated herein by way of reference material.
A problem with display devices of this type arises from the fact that they have current-addressed display elements. Conventional supply circuitry for supplying a controllable current to the display elements can suffer the drawback that the current varies as a function of the electrical characteristics of the switching transistors used in the supply circuitry. For example, a current controlling transistor may be provided as part of the pixel configuration, with the gate voltage supplied to the transistor determining the current through the display element. Different transistor characteristics give rise to different relationships between the gate voltage and the source-drain current. Such an arrangement is described in EP-A-0653741.
The current controlling circuitry may either comprise part of the pixel configuration, as described above, so that a pixel voltage is supplied to the pixels, or else the current controlling circuitry may comprise separate circuitry provided at the periphery of the display area, so that a pixel current is supplied to the pixels. In either case, if the current controlling circuitry is integrated onto the same substrate as the display pixels, it typically comprises thin film switching elements such as thin film transistors. The uniformity across the substrate of the electrical characteristics of the switching elements may be poor, which gives rise to unpredictable variations in the pixel currents and therefore the pixel outputs.
According to the invention, there is provided a display device comprising:
an array of pixels arranged in rows and columns, each pixel comprising a current-addressed display element;
driver circuitry for generating current signals corresponding to desired outputs from the display elements, the driver circuitry comprising a transistor switching device for applying a charging voltage to a switched capacitor arrangement, which comprises a capacitor and a switch arrangement enabling the capacitor to be selectively charged and discharged at a predetermined rate to the charging voltage,
wherein a transistor control voltage is applied to a control terminal of the transistor switching device so as to provide the charging voltage to the switched capacitor arrangement, and wherein the transistor control voltage is adjusted depending on the transistor threshold voltage thereby to ensure that the capacitor is charged to the charging voltage irrespectively of the value of the threshold voltage.
The driver circuitry used in the display device of the invention enables an accurately controllable current to be provided which is used to drive the current-addressed pixels. The circuit is implemented using capacitors and transistors and can therefore be integrated onto the display device active plate, and variations across the plate giving rise to transistor threshold variations are compensated.
A sampling circuit may be provided for adjusting the transistor control voltage, the sampling circuit comprising a switch arrangement and a threshold capacitor, the sampling circuit being operable in a first mode to charge the threshold capacitor to the transistor threshold voltage and in a second mode to add the transistor threshold voltage stored on the threshold capacitor to a transistor control voltage.
The threshold voltage of the transistor is thus measured and compensated by storing charge on a threshold capacitor.
The switched capacitor arrangement may comprise a first pair of switches and first associated capacitor, and a second pair of switches and second associated capacitor, wherein the switches are operated to provide charging of one capacitor simultaneously with discharging of the other capacitor. This enables a continuous charging current to be drawn by the switched capacitor arrangement, which reduces the current ripple of the current supply.
The switched capacitor arrangement may also comprise a column capacitor which is charged during an initial operation period of the driver circuitry. This enables compensation for the column capacitance of a column of pixels at the start of the current generation cycle, so that the circuit stabilizes more rapidly.
Instead of sampling the threshold voltage, the adjusted transistor control voltage may be provided by the output of a differential amplifier, with one of the amplifier inputs being supplied with the non-adjusted transistor control voltage and the other of the amplifier inputs being supplied to the switched capacitor arrangement as the charging voltage.
Preferably each pixel comprises an electroluminescent display element, and each pixel may comprise first and second switching means, operable in a first mode in which the input current is supplied by the first switching means to the second switching means, a control level being stored for the second switching means corresponding to the input current, and in a second mode in which the stored control level is applied to the second switching means so as to drive a current corresponding to the input current through the display element.